The invention relates generally to semiconductor fabrication and more specifically to in-line metrology for process control during wafer processing.
During semiconductor fabrication, in-line and in-situ metrology for process control and/or verification is commonly used. Inductive sensors, frequently referred to as eddy current sensors (ECS), were recently introduced for semiconductor metrology for metal film thickness monitoring. Materials introduced from the alternative to the eddy current sensor side of the film helped to enhance dramatically sensor sensitivity especially in the thin film range, thereby allowing ECS to perform real-time measurements both in-situ by utilizing wafer carrier built-in sensors, as well as stand-alone units for 3-CMP wafer characterization.
However, one shortcoming with ECS at the enhanced sensitivity mode is that the silicon substrate, as well as numerous other conductive objects located within the ECS sensing vicinity, contribute to the total signal. Consequently, the interpretation of the ECS readings introduce significant uncertainty, especially in the low thickness range, which places limitations upon the measurement capabilities especially for ultra-thin reasonably high resistivity diffusion barrier films, i.e., tantalum and tantalum nitride barrier films. Furthermore, the eddy currents generated within the thin film and within the substrate are inductively coupled. This inductive coupling seriously affects the thin film lower sensitivity.
In view of the foregoing, there is a need to provide a method and apparatus that is capable of monitoring the barrier film thickness in order to provide an in-line metrology device capable of providing accurate thin metal film thickness.